Accepted Manuscript Title: Microbially-driven strategies for bioremediation of bauxite residue Author: Talitha C. Santini Janice L. Kerr Lesley A. Warren PII: S0304-3894(15)00212-5 DOI: http://dx.doi.org/doi:10.1016/j.jhazmat.2015.03.024 Reference: HAZMAT 16672 To appear in: Journal of Hazardous Materials Received date: 9-12-2014 Revised date: 12-2-2015 Accepted date: 12-3-2015 Please cite this article as: Talitha C.Santini, Janice L.Kerr, Lesley A.Warren, Microbially-driven strategies for bioremediation of bauxite residue, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.03.024 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Microbially-driven strategies for bioremediation of bauxite residue Talitha C. Santini1,2,3, *, Janice L. Kerr1, Lesley A. Warren4 1 Centre for Mined Land Rehabilitation, Sir James Foots Building, The University of Queensland, St Lucia QLD 4072, Australia 2 School of Geography, Planning, and Environmental Management, Steele Building, The University of Queensland, St Lucia QLD 4072, Australia 3 School of Earth and Environment, The University of Western Australia, 35 Stirling Hwy, Crawley WA 6009, Australia 4 School of Geography and Earth Sciences, McMaster University, 1280 Main Street West, Hamilton ON L8S 4K1, Canada * corresponding author: [email protected]; phone: +61 7 3346 1467; fax: +61 7 3365 6899 Abstract Globally, 3 Gt of bauxite residue is currently in storage, with an additional 120 Mt generated every year. Bauxite residue is an alkaline, saline, sodic, massive, and fine grained material with little organic carbon or plant nutrients. To date, remediation of bauxite residue has focussed on the use of chemical and physical amendments to address high pH, high salinity, and poor drainage and aeration. No studies to date have evaluated the potential for microbial communities to contribute to remediation as part of a combined approach integrating chemical, physical, and biological amendments. This review considers natural alkaline, saline environments that present similar challenges for microbial survival and evaluates candidate microorganisms that are both adapted for survival in these environments and have the capacity to carry out beneficial metabolisms in bauxite residue. Fermentation, sulfur oxidation, and extracellular polymeric substance production emerge as promising pathways for bioremediation whether employed individually or in combination. A combination of bioaugmentation (addition of inocula from other alkaline, saline environments) and biostimulation (addition of nutrients to promote microbial growth and activity) of the native community in bauxite residue is recommended as the approach most likely to be successful in promoting bioremediation of bauxite residue. Keywords Bioremediation; bauxite residue; bioaugmentation; biostimulation; geomicrobiology Abbreviations 1 ADP: adenosine diphosphate; ATP: adenosine triphosphate; EC: electrical conductivity; EPS: extracellular polymeric substance; ESP: exchangeable sodium percentage 1. Introduction 1.1 Production and management of bauxite residue In 2013, 259 Mt of bauxite was mined globally, the largest production being from Australia [1] (Figure 1). For every metric tonne of aluminium metal produced from bauxite, two tonnes of bauxite residue (also referred to as red mud or alumina refining tailings) are generated [2]. After more than 110 years of commercial aluminium production, bauxite residue storage facilities worldwide currently hold an estimated 3 Gt of residue and are increasing by approximately 120 Mt per year [3]. The large and continually growing mass of stored bauxite residue highlights the need for effective remediation strategies to manage the environmental impacts of aluminium production and contribute to industry sustainability. [Suggested location for Figure 1] As a byproduct of the Bayer process used for alumina refining, bauxite residue is a highly alkaline (pH ≈ 11.3), saline (electrical conductivity ≈ 7.4 mS cm-1), sodic (exchangeable sodium percentage ≈ 69 %), massive (bulk density ≈ 2.5 g cm-3), and fine grained (specific surface area ≈ 32.7 m2 g-1) tailings material [4]. Bauxite residue pore water is dominated by the cations Na+ + 2+ 2+ - 2- 2- - (major), K , Ca and Mg , and the anions Al(OH)4 , SO4 , CO3 , and OH [5]. Major minerals present in bauxite residue include a mixture of residual minerals from the parent bauxite (hematite, goethite, quartz, kaolinite, anatase, rutile, undigested gibbsite, boehmite, or diaspore) as well as precipitates formed during the Bayer process (perovskite, calcite, tricalcium aluminate, and zeolitic desilication product minerals such as sodalite and cancrinite) [4]. With the exception of perovskite, Bayer process precipitate minerals dissolve slowly during rainfall leaching and weathering of bauxite residue and release salts (Na+, Ca2+, various anions depending on mineral 2- - composition) and alkalinity (in the form of CO3 and OH ) to pore water solutions, maintaining the high pH and salinity of bauxite residue over time [4, 6]. The chemical and physical properties of bauxite residue pose significant challenges for remediation, the aim of which is to establish and maintain a vegetation cover after closure of tailings storage facilities, and convert the land area occupied by tailings to an alternative use. This requires transforming bauxite residue from tailings to a soil material with chemical, physical, and biological properties similar to those of productive forest and grassland soils (Figure 2). Specific remediation physico-chemical targets include [7]: pH between 5.5 and 9.0; electrical conductivity < 4 mS cm-1; exchangeable sodium percentage < 9.5 %; and bulk density of < 1.6 g cm-3. These are all values typical of ranges observed in well- functioning soils derived from bedrock parent materials. Without targeted strategies, natural weathering processes will drive remediation and soil processes as shown in the conceptual model of soil formation in Figure 2. Chemical, physical, and biological amendments as depicted in 2 Figure 2 offer opportunities to accelerate soil formation rates and thus achieve the same remediation goals in a shorter timeframe. [Suggested location for Figure 2] Applications of various chemical and physical amendments to accelerate remediation and soil development in bauxite residue have been studied for nearly four decades [6, 8-16] (Figure 2). The focus of these studies was to lower salinity, sodicity, and alkalinity, and to encourage structure development. The application of gypsum, combined with tillage and irrigation, addresses several of these targets simultaneously, by providing a source of Ca2+ to displace Na+, as well as facilitating export of alkaline, saline-sodic pore water [7, 17, 18], and development of stable soil structure [18]. Plants grown in amended residues had increased seedling emergence rates and dry weight [15]. Organic amendments such as hay have also been successful in improving soil properties and encouraging the development of microbial and plant biomass [19]. Combinations of inorganic and organic amendments have also been evaluated. In combination with gypsum, mushroom compost and sewage sludge were also effective in improving soil chemistry and structure and in promoting plant growth [9, 11, 18-20]. In addition to providing a carbon source and improving structure and drainage, organic amendments such as compost and topsoil may act as microbial inoculants, providing a viable microbial community to perform vital soil ecosystem functions such as nutrient cycling [21], but this has not yet been investigated (Figure 2). The roles of microbial communities in contributing to remediation goals have been largely ignored in research to date, with research confined to an examination of the responses of any extant microbial community to remediation strategies [21, 22] rather than evaluating the extent to which these microbial communities play active roles in remediation. Furthermore, no studies have evaluated the survivability of microbial inoculants in bauxite residue, or factors which influence survival and the extent to which microbial inoculants may influence remediation of bauxite residue. 1.2 Microbial community responses and roles in remediation Microorganisms are important components of all ecosystems, playing a vital role in soil development (pedogenesis) and in supporting plant nutrition (Figure 2). Interactions between microbial communities and the environments which they inhabit are often difficult to disentangle as they influence one another in complex ways. For example, environmental conditions influence microbial community structure and function by imposing selection pressures and regulating supply of substrates and removal of products, and microbial communities in turn influence environmental conditions through the reactions of their metabolic products with components of the surrounding environment. Investigation of the feedbacks between environmental conditions and microbial communities, especially in a geochemically dynamic and extreme environment 3 such as tailings, therefore requires careful